Cook's note: Use good quality chocolate.
Preheat the oven to 350 degrees F. Grease 2 (9-inch) round cake pans. (If silicon: place on a metal baking sheet. If metal: line with waxed paper.).
As the oven preheats, begin to make the cake batter. Dissolve the cocoa into the espresso. Stir the buttermilk into the cocoa mixture and set aside to cool. Whisk together the flour, baking soda and salt. In another bowl, beat the butter with an electric mixer at medium speed for about 6 minutes, while gradually adding the sugar. (Use the paddle attachment for a stand mixer.) The butter mixture should be light and fluffy. Slowly add the eggs and vanilla to the butter, while continuing to beat. Be sure to scrape down the sides of the bowl as you go, so all the ingredients get incorporated. Fold the flour and cocoa mixtures into the butter mixture with a rubber spatula, alternating from flour to cocoa in the following order: 1/3 flour mixture; 1/2 cocoa mixture; 1/3 flour mixture; 1/2 cocoa mixture; 1/3 flour mixture. Do not overmix!
Pour half of the finished batter into each of the cake pans and level the tops with the spatula. Bake for 30-40 minutes in the center of the oven, rotating halfway through. Check with a wooden toothpick for doneness. Cool for at least 15 minutes before inverting onto a baking rack to cool completely.
Make the buttercream: With an electric mixer beat the peanut butter and butter in a large bowl until creamy. Add the powdered sugar and beat until spreadable.
With a serrated knife halve the two layers making 4. Place 1 cake layer top down on the rack and spread 1/3 of the buttercream to the edges of the layer. Top the first cake layer with another and repeat with the buttercream. Continue layering with the remaining layers and buttercream, ending with cake. (I frosted the outside as well, but wasn't noted in recipe.) Refrigerate covered in plastic wrap for at least 3 hours and up to overnight.
Remove the cake from the refrigerator and carefully transfer to a wire rack over a sheet pan. (Line the space under the rack with some old newspaper, for easy cleanup.).
To make the ganache, chop the butter and scatter it into the chocolate pieces in a large bowl. Heat the cream and corn syrup in a saucepan over moderate heat. As soon as the cream mixture begins to boil, pour it over the chocolate mixture and begin stirring vigorously. When the chocolate has melted and is combined thoroughly with the cream, stir in the vanilla. Quickly begin pouring the ganache over the top of the cake, allowing it to flow down and cover the sides. (You may use a metal frosting spatula to smooth the top and sides.) Pack the chopped peanuts onto the ganache to cover the sides of the cake, leaving the chocolate on top exposed. Store in the refrigerator.
Serving suggestions: I like to reserve a little of the buttercream and a handful of whole honey roasted peanuts to decorate the top of the cake. Try using a star tip to pipe points of buttercream in a circle, bordering the top of the cake. Then, set peanuts into the buttercream border to form an interesting design.